There are several types of heat exchanger:
Recuperative type, in which fluids exchange heat on either side of a dividing wall
Regenerative type, in which hot and cold fluids occupy the same space containing a matrix of material that works alternatively as a sink or source for heat flow
Evaporative type, such as cooling tower in which a liquid is cooled evaporatively in the same space as coolant.
The recuperative type of heat exchanger, which is the most common in practice, may be designed depending on their flow arrangement. In parallel-flow heat exhangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side. In counterflow heat exchangers, which are often more efficient, the fluids enter the exchanger from opposite ends. In a cross-flow heat exchanger, the fluids travel roughly perpendicular to one another through the exchanger.
The most basic and the most common type of heat exchanger construction is the tube and shell. This type of heat exchanger consists of a set of tubes in a container called a shell. The fluid flowing inside the tubes is called the tube side fluid and the fluid flowing on the outside of the tubes is the shell side fluid. At the ends of the tubes, the tube side fluid is separated from the shell side fluid by the tube sheet(s). The tubes are rolled and press-fitted or welded into the tube sheet to provide a leak tight seal. In systems where the two fluids are at vastly different pressures, the higher-pressure fluid is typically directed through the tubes and the lower pressure fluid is circulated on the shell side. This is due to economy, because the heat exchanger tubes can be made to withstand higher pressures than the shell of the heat exchanger for a much lower cost. The support plates act as baffles to direct the flow of fluid within the shell.
While shell and tube heat exchanger are widely in use, they are being replaced by new exchangers as:
They are generally large and heavy
Require generous space for mounting.
Lower heat transfer coefficients
Another type of heat exchangers widely used are heat exchangers in which heating elements are formed from straight (uncoiled) segments of round or square heating cable. Helical coil heat exchangers offer distinct advantages, such as improved thermal efficiency, compactness, easy maintenance and lower installed cost. When an application requires equipment suitable for high operating pressure and/or extreme temperature gradients, a helical coil unit is considered. The exchangers also are suitable for less demanding applications, such as heat recovery, condensing, boiling and basic heat exchange.
Although various configurations are available, the basic and most common design consists of a series of stacked helically coiled tubes. The tube ends are connected to manifolds, which act as fluid entry and exit locations. The tube bundle is constructed of a number of tubes stacked atop each other, and the entire bundle is placed inside a casing, or shell. To effectively optimize thermal and hydraulic requirements, the number of tubes (coils) along with their spacing and length may be varied. This allows a design to meet the thermal and hydrodynamic requirements of both the casing and tube side fluids, so users can select a unit that matches their application's thermal demand and hydraulic limitations.
As with any heat exchanger, the flow rate, allowable pressure, physical properties of the fluid, and construction material control final design. High film coefficients are achieved on both the coil and casing side. The helical flow path imparts higher shear rates and turbulence at a given pressure drop, which can result in film coefficients up to 40% higher than those achieved with many comparable shell and tube units. The spiral pattern also promotes turbulence, leading to increased heat transfer rates. In addition, there are no baffles or dead spaces that lead to inefficiencies commonly found in other types of shell and tube exchangers. The net result is a Helical coil Heat Exchanger that is up to 40% more efficient than a standard shell and tube.
A helical coil heat exchanger is easy to maintain. The casing of the unit can be removed without disturbing any of the piping connections. Once the casing is removed, the entire tube bundle is exposed for inspection. With the casing removed, the shell side of the unit can easily be cleaned in place.
Originally built for use in boiler sample cooling over 60 years ago, there are thousands of helical coil heat exchangers being used today in hundreds of services. Many units have been in operation for well over 40 years. The service life varies with the application, but its many features add to its reliability when compared to a shell and tube exchanger.
No gaskets are required for the tube side of the helical coil heat exchanger. Aggressive fluids are often placed tube side for this reason. No gaskets on the tube side will minimize the chance of leakage.
The spring-like coil of the helical coil heat exchanger reduces stresses caused by thermal expansion of the tube material.
Helical coil heat exchanger can do the job in a fraction of the space required by typical straight shell and tube exchangers. With higher heat transfer efficiencies, the surface area required is normally less than a straight shell and tube. Smaller surface requirements, and the coiled tube design result in a very compact unit. Access space required for maintenance or inspection is very small compared to straight shell and tube exchangers. The only space required for a helical coil heat exchanger is to remove the casing, which allows inspection of both the entire tube bundle and shell side of the exchanger. One can mount a helical coil heat exchanger on columns, nozzles, walls, ceilings, or in-line; and require no support.
Careful selection of the helical coil heat exchanger can accommodate low flow rates while maintaining reasonable velocities.
High (or low) temperature fluids will cause the tube material to expand (or contract). This thermal expansion creates high stresses at the tube-to-tube sheet joints of standard straight shell and tube type units. The helical coil heat exchanger, with its spring-like coils, allows movement of the coil within the bundle. This feature allows the heat exchanger to easily withstand temperature differences of over 500 degrees F. between fluids. Cyclic operation and extreme temperatures are easily handled. Helical coil heat exchanger can be built to accommodate up to 15,000 psig design pressures. The helical coil heat exchanger is capable of thermosiphon when mounted in the correct orientation.